A physical quantity sensor detects a physical quantity by measuring a capacitance change of a capacitor between a fixed electrode supported on a substrate and a movable electrode disposed on a beam, which is displaced by a physical quantity.
The physical quantity sensor is, for example, used for a vehicle. The sensor, for example, works as an acceleration sensor for controlling kinetic control equipment such as a vehicle stability control system. This acceleration sensor is required to have high reliability and to be minimized. Therefore, the sensor is manufactured by using a micro machine process, to which a conventional semiconductor manufacturing process is applied. The acceleration sensor is disclosed, for example, in Japanese Unexamined Patent Application Publication No. 2002-71707 (i.e., U.S. Pat. No. 2002-0,023,492). The sensor is a capacitance type acceleration sensor, which includes a support substrate, a beam, a movable electrode and a fixed electrode.
The support substrate is made of silicon substrate. The beam is made of silicon, and supported on the substrate through an anchor and an insulation film. The movable electrode has a comb teeth structure, and is formed integrally with the beam. The fixed electrode is made of silicon, has a comb teeth structure, and is supported on the substrate through the insulation film.
The support substrate has a principal plane having a plate shape. When the acceleration including a certain component is applied to the sensor, the movable electrode is displaced in accordance with the acceleration. Therefore, a distance between the movable electrode and the fixed electrode is changed, so that the capacitance of a capacitor between the fixed and movable electrodes is also changed. Then, the sensor detects the capacitance change and outputs a sensor signal corresponding to the acceleration. Here, the certain component of the acceleration is parallel to the principal plane of the substrate and perpendicular to a protrusion direction of the movable and fixed electrodes.
When the sensor is mounted in the vehicle, and an electronic substrate for accommodating the sensor is horizontalized in the vehicle, the sensor is mounted parallel to the electronic substrate so that the acceleration parallel to the horizontal direction (i.e., the direction parallel to the ground) can be detected. However, when the sensor detects the acceleration perpendicular to the horizontal direction (i.e., the direction perpendicular to the ground), the sensor is required to be mounted perpendicularly to the electronic substrate. Therefore, an additional handling jig is necessitated to mount the sensor on the electronic substrate in such a manner that the sensor is perpendicular to the electronic substrate. This cause a manufacturing cost of the sensor higher. Further, two acceleration sensors are mounted on the substrate so as to detect the accelerations in two directions, one of which is perpendicular to the other. In this case, it is necessary to mount two sensors having a plate shape in a package and to orthogonalize the sensors each other. Therefore, arrangement efficiency of the package is decreased so that the package becomes larger. Thus, minimization of the package cannot be achieved.
Further, when the sensor is minimized, the distance between the fixed and movable electrodes is also reduced. Therefore, the movable electrode is affected by a coulomb force generated between the fixed and movable electrodes. The coulomb force is in proportion to the distance squared. Therefore, a relationship between the acceleration applied to the sensor and the sensor output shows a non-linear characteristic. That is, the relationship does not show a proportionality relation. Therefore, an available range of the sensor output is limited.